1. Field of the Invention
The present invention relates to a fiber type optical wavelength converter device, which converts a fundamental wave into a second harmonic having a wavelength which is 1/2 of that of the fundamental wave, and an optical wavelength converter module employing such an optical wavelength converter device.
2. Description of the Prior Art
Various attempts have heretofore been made to convert the wavelength of a laser beam into a second harmonic, e.g., shortening the wavelength of a laser beam, or using a nonlinear optical material. One well known example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk crystal type converter device as disclosed, for example, in Introduction to Optical Electronics, pages 20.about.204, written by A. Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K.K.). This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence cannot be employed, even if it has high nonlinearity.
To solve the above problem, there has been proposed a fiber-type optical wavelength converter device. An optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of a nonlinear optical material and surrounded by a cladding. One example of such an optical fiber is shown in Vol. 3, No. 2, pages 2818 32, of the Bulletin of the Microoptics Research Group of a gathering of the Applied Physics Society. Recently, much effort has been directed to the study of a fiber-type optical wavelength converter device since it can easily achieve phase matching between a guided mode, in which a fundamental is guided through the core, and a radiated mode, in which a second harmonic is radiated into the cladding (for so-called Cerenkov radiation).
The wavelength-converted wave produced by the fiber-type optical wavelength converter device is emitted from the output end of the cladding and utilized in various applications. In many applications, the wavelength-converted wave is converged into a small spot. When a wavelength-converted wave is used to record optical signals, for example, the wavelength-converted wave is focused into a very small spot so that higher recording density can be achieved.
However, it has been recognized that the wavelength-converted wave emitted from the fiber Cerenkov-type optical wavelength converter device cannot be converged into a small spot even if the emitted wave is passed through a general spherical lens. In view of this drawback, the inventors of the present application have proposed an optical wavelength converter device capable of converging a wavelength-converted wave into a small spot and an optical wavelength converter module employing such an optical wavelength converter device (see U.S. patent application Ser. No. 384,532). The proposed optical wavelength converter device comprises a fiber Cerenkov-type optical wavelength converter device which includes a cladding having an exit end for emitting a wavelength-converted wave. The exit end of the cladding may have a conical shape for converting the conical wavefront of the wavelength converted wave into a plane wavefront; alternatively it may be shaped like an aspherical lens for converting the conical wavefront into a spherical wavefront, or it may support a grating composed of concentric patterns for converting the conical wavefront into a plane or spherical wavefront.
The proposed optical wavelength converter module comprises a fiber Cerenkov optical wavelength converter device of the type described above and an optical device disposed such that it receives a wavelength-converted wave emitted from the exit end of the cladding of the optical wavelength converter device and converts the conical wavefront of the received wavelength-converted wave into a plane or spherical wavefront.
If the wavefront of a wavelength-converted wave can be converted into a plane wavefront by the above optical device, by a conically-shaped exit end of the cladding, or by a concentric pattern grating on the exit end of the cladding, then the wavelength-converted wave can be focused into a small spot by being passed through an ordinary spherical lens.
If the wavefront of a wavelength-converted wave can be converted into a convergent spherical wavefront by the above optical device, by an exit end of the cladding shaped like an aspherical lens, or by a concentric pattern grating on the exit end of the cladding, then the wavelength-converted wave can be focused into a small spot. Even if the wavefront of a wavelength-converted wave is converged into a divergent spherical wavefront, the wavelength-converted wave can be focused into a small spot by being passed through an ordinary spherical lens.
In order for the wavelength-converted wave to focus well, the length of the fiber, the diameter of the core, and the distance between the optical wavelength converter device and the optical device must be strictly set to certain values corresponding to the wavelength of the fundamental. Therefore, when the optical wavelength converter devices and the optical wavelength converter modules are fabricated, the tolerances for the fiber length, the core diameter, and other dimensions are very small. Therefore, optical wavelength converter devices and modules of the above type are manufactured with a low yield and hence are expensive. When an optical wavelength converter device or module is employed, the light source, such as a semiconductor laser used in combination therewith as a fundamental wave generator, should not produce large variations or fluctuations in the wavelength. Consequently, a system which employs the optical wavelength converter device or module is also expensive.
With the present optical fiber fabricating technology, it is highly difficult or even impossible to produce a core, for example, such that any error in its diameter will be held within the specified limited tolerance.
There are certain instances in applications which do not involve wavefront conversion as described above where the tolerances specified for the length of a fiber, the diameter of a core, and other dimensions are also extremely small. In such cases, the optical wavelength converter device or module is costly or even impossible to fabricate due to limits in the capabilities of the fabricating technology.